The Use of Ceramics in Lithium Battery Separators
In lithium batteries, the separator allows for ion conduction but not electron conduction.
It serves to isolate the positive and negative electrode materials, preventing direct contact and short-circuits. At the same time, it affects the transport of Li+ ions between the positive and negative electrode materials, thereby influencing the materials’ cycling and rate performance.
Polyolefin separators are the current mainstream separators, but they exhibit poor thermal stability.
The melting points of polypropylene (PP) and polyethylene (PE) are 165°C and 135°C, respectively, which can lead to potential safety issues. Because at high temperatures, the separator can shrink or melt, causing internal short circuits, leading to fires or even explosions.
Lithium-ion Battery Separators (Source: Kang Le et al., “Research Progress on Ceramic Separator Materials for Lithium-ion Batteries”)
Ceramics in Lithium Battery Separators
In response to the above situation, various methods have been employed to enhance the thermal stability of separators. Among these, coating a layer of inorganic ceramic particles on PP or PE separators is considered the most effective and economical approach.
Ceramic materials provide high heat resistance, while adhesives offer adhesion to maintain the integrity of the coating and the entire composite separator structure.
On one hand, by improving thermal stability, these ceramic-coated separators can effectively enhance the safety of lithium-ion batteries by preventing short circuits at high temperatures.
On the other hand, ceramic-coated separators demonstrate excellent wetting and liquid retention capabilities with the electrolyte and positive/negative electrode materials, significantly improving battery performance and lifespan.
Types of Ceramic Coatings
Alpha-alumina is an inorganic oxide with high thermal stability and chemical inertness, offering excellent high-temperature resistance that significantly enhances the safety of lithium-ion batteries.
Alumina coatings also have the advantage of neutralizing free HF in the electrolyte, thereby extending the battery’s lifespan. Therefore, alpha-alumina is considered one of the best choices for lithium-ion battery separator coatings.
Furthermore, surface modification of alumina is achieved by adding water-soluble anionic polymers during production, forming a stable double layer structure on its surface.
This process involves adsorbing hydroxyl and carboxyl functional groups, increasing the surface potential of alumina particles and creating steric hindrance.
This improvement in particle dispersion enhances the stability of ceramic slurry.
PE-Based Film SEM and Alumina-Coated Film SEM
Comparison of Physical Properties between PE-Based Film and Alumina-Coated Film (Source: Liu Tianyi et al., ‘Research Progress on High-Temperature Ceramic-Coated Lithium-ion Battery Separators’)
NO. | Item | Typical Values | |
Typical Values for Base Film | Typical Values for Alumina-Coated Film | ||
1 | Thickness/μm | 9.1 | 12.2 |
2 | Areal Density/ (g/m2) | 5.101 | 9.956 |
3 | Tensile Strength (MPa) Longitudinal / Transverse | 227/244 | 178/184 |
4 | Elongation (%) Longitudinal / Transverse | 107/86 | 100/80 |
5 | Puncture Strength（gf） | 484 | 478 |
6 | Heat Shrinkage (%) Longitudinal / Transverse (120°C, 1h) | 4.43/3.94 | 1.48/0.44 |
7 | Heat Shrinkage (%) Longitudinal / Transverse (130°C, 1h) | 15.58/15.72 | 1.69/0.99 |
8 | Permeability (Sec/100cc) | 143 | 176 |
9 | Peel Strength / (N/cm) | — | 0.718 |
Boehmite, also known as monohydrate alumina or pseudoboehmite, with a molecular formula of γ-AlOOH, is primarily synthesized through the hydrothermal process of aluminum hydroxide. When used as a ceramic coating for lithium-ion battery separators, boehmite exhibits a uniform polyhedral particle structure.
Its low hardness results in minimal mechanical wear during cutting and coating processes, reducing equipment wear and the risk of introducing foreign matter.
Additionally, boehmite has a lower density, covering 25% more area by weight than α-alumina.
As the manufacturing processes become more mature and boehmite gains recognition in the market, its share in the ceramic separator field has been increasing year by year.
Silicon dioxide is a low-cost and environmentally friendly compound widely used in the electronics industry.
Silicon dioxide is one of the most extensively researched coating materials apart from α-alumina and boehmite.
Furthermore, various other ceramic materials such as CeO2, MgAl2O4, ZrO, TiO2, and others are also extensively researched. Ceramic separators made using these materials exhibit excellent thermal stability and outstanding wettability with electrolytes.
Ceramic Coating Process
Ceramic-coated separators typically use PP, PE, or multilayer composite separators as the base material. Through a specific coating process, a ceramic layer is applied to the surface of the base material. After coating, the ceramic tightly bonds to the base material.
To produce well-performing ceramic-coated separators, it is often necessary to add binders, wetting agents, thickeners, dispersants, leveling agents, and other additives to the slurry.
- Binders are primarily used to enhance the adhesion strength between ceramic powders and the substrate, and acrylic ester polymers are commonly used at present. l
- Wetting agents reduce the interfacial tension of the separator, allowing the slurry to spread out on the separator base. l
- Thickeners are used to increase the viscosity of the slurry, alter its physical properties, prevent sagging during the coating process, and stabilize the slurry for improved storage performance. l
- Dispersants promote the even dispersion of ceramic particles in the medium, forming a stable suspension. l
- Leveling agents facilitate the formation of a smooth, uniform, and even coating during the drying and film-forming process.
To enhance the uniformity of ceramic coatings and improve the engineering capabilities of the coating process, various ceramic coating techniques have been developed, including wire-wound rod coating, reverse roll coating, gravure roll coating, slot-die extrusion coating, and ribbon slot extrusion coating.
Comparison of Coating Processes for Ceramic-Coated Lithium-Ion Battery Separators (Source: Cheng Rui et al., ‘Application of Ceramics in Liquid Lithium-Ion Battery Separator Materials’)
Coating Processes | Viscosity/cp | Wet Thickness/μm | Coating Error/% | Features |
Wire-Wound Rod Coating | 0.020 -1.000 | 5-50 | 10 | Poor Coating Uniformity, Non-Uniform |
Reverse Roll Coating | 0.010-50.000 | 5-400 | 5 | Improved Coating Uniformity Compared to Wire-Wound Rod Coating |
Gravure Roll Coating | 0.001-5.000 | 1-25 | 2 | Gravure Roll Wear, Wastage of Adhesive, Uniform Coating, No Wrinkles, etc. |
Slot-Die Extrusion Coating | 0.005-0.500 | 15-250 | 2 | Thicker Coating with Good Uniformity |
Ribbon Slot Extrusion Coating | 0.005-20.00 | 2-250 | 2 | Thin Coating, High Precision, High Cost |
Single-Sided or Double-Sided Coating?
①Using a three-layer composite film of polypropylene (PP)/PE/PP as the base film, the impact of single-sided and double-sided ceramic coating on the performance of 18650-type LiNi0.8Co0.15Al0.05O2/C lithium-ion batteries was studied.
② The physical properties of the separator, such as micropore morphology, air permeability, and ionic conductivity, were analyzed.
③ The effect of the separator on the battery’s electrochemical performance was investigated and compared with batteries using the base film.
The results of the above study showed that ceramic coating on the separator surface results in different micropore structures, air permeability, and ionic conductivity.
The air permeability values for composite separators, single-sided coating, and double-sided coating are 501s/100ml, 220s/100ml, and 175s/100ml, respectively. The ionic conductivity values are 0.115mS/cm2, 0.312mS/cm2, and 0.385mS/cm2, respectively.
Batteries made with double-sided ceramic-coated separators exhibit higher air permeability and ionic conductivity, thus providing optimal rate performance.
The 5.00C rate discharge capacity of batteries with double-sided ceramic-coated separators is 85.13% of the 0.20C discharge capacity.
The results of battery charge retention and cycle performance tests show that the capacity retention rates for the base film, single-sided ceramic-coated separator, and double-sided ceramic-coated separator are 96.84%, 97.35%, and 98.09%, respectively.
When cycled at a rate of 2.00C for 300 cycles, the capacity retention rates for the base film, single-sided ceramic-coated separator, and double-sided ceramic-coated separator are 88.59%, 93.97%, and 94.47% of the initial capacity, respectively.
Double-sided ceramic-coated separators also improve the battery’s charge retention and cycle performance.
Overall, the comprehensive assessment of the battery’s performance indicates that double-sided ceramic-coated separators exhibit good electrochemical performance.
In Summary
The separator is one of the key internal components with significant technological barriers in the lithium-ion battery industry chain.
To enhance the high-temperature resistance, heat shrinkage resistance, and mechanical strength of separators, thereby improving battery safety, a ceramic layer can be applied to the surface of the separator. This leverages the excellent heat stability and mechanical strength of ceramic materials to enhance the safety of lithium-ion battery separators.
Furthermore, ceramic layers typically feature abundant pore structures and good electrolyte wettability, which can enhance the liquid absorption and retention capabilities of separators, significantly extending the battery’s lifespan.
Moreover, ceramic layers often utilize non-polar oxides, which can neutralize small amounts of hydrofluoric acid in the electrolyte and suppress cell expansion.
The material, morphology, particle size, and the choice of adhesive for ceramic materials can all impact the performance of ceramic-coated separators. Additionally, the pore structure and thickness of the coating layer play a crucial role in the performance of the separator.
Therefore, the rational optimization of ceramic materials, adhesives, and the selection of appropriate coating processes to prepare ceramic-coated separators with uniform thickness and a reasonable pore structure, while considering cost and the reliability for engineering applications, will be a key focus of future research and development efforts.
However, the conversion of ceramic materials into films of a certain thickness for direct use as separators in lithium-ion batteries still presents challenges related to mechanical strength, pore structure, process feasibility, and reliability.